1. Field of the Invention
The invention relates generally to the field of oscillators. More particularly, the invention relates to voltage controlled oscillators.
2. Discussion of the Related Art
A voltage-controlled oscillator (VCO) is a circuit that generates an oscillating signal at a frequency proportional to an externally applied control voltage. These types of circuits find several applications in telecommunications, and are useful for tracking and matching signal frequencies as they shift due to thermal variations, power supply fluctuations, and other sources of frequency shifts.
Modern electronics often require a VCO to operate over large frequency ranges. Nevertheless, increasing the tuning bandwidth often degrades the signal-to-noise ratio (SNR) of the VCO""s output. While multi-band products can use multiple VCO""s for multiple frequency ranges, this adds to chip area and require undesirable switching of signal paths.
U.S. Pat. No. 3,813,615 to Okazaki describes an oscillator circuit suitable to operate at low and high band frequencies by switching of an inductance. While an inductance switching VCO may tend to keep a constant tuning range, it is not practical given current integrated circuit (IC) technology.
Capacitance can be switched by changing the voltage across a tuning element such as a voltage variable capacitor (VVC). Nevertheless, the more capacitance is switched in for operation in lower frequency ranges, the more the tuning range decreases. Thus, compromises have to be made between tuning tolerance on the low range and noise on the high range.
Until now, the requirements of providing a method and/or apparatus for a band switched voltage controlled oscillator with constant tuning range that is suitable for use with current IC technology have not been met.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
According to an aspect of the invention, a method for operating a band switchable voltage controlled oscillator in at least two different frequency bands of substantially equal bandwidth comprises: operating said voltage controlled oscillator in a first frequency band by switching a first capacitive circuit having a capacitance that varies with a tuning voltage; and operating said voltage controlled oscillator in a second frequency band by switching a second capacitive circuit having a capacitance that does not vary with the tuning voltage. According to another aspect of the invention, a band switchable voltage controlled oscillator, comprises: a switchable variable capacitance circuit; a switchable fixed capacitance circuit coupled to the switchable variable capacitance circuit; a controller for selectively switching said switchable fixed and variable capacitance circuits; a fixed tank capacitance circuit coupled to the switchable fixed capacitance circuit; a main tuning voltage variable capacitance circuit coupled to the fixed tank capacitance circuit; a tank inductance coupled to the main tuning voltage variable capacitance circuit; and an amplifier circuit coupled to the tank inductance.
These, and other, embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements.